1. Field of the Invention
The present invention relates, generally, to water quality monitors and, more particularly, to quality monitors for reverse osmosis water purification apparatuses.
2. Description of the Related Art
As the public becomes more aware of the potential environmental and health hazards associated with our municipal water supply sources, water purification apparatuses have experienced an increase in commercial and residential use. In many rural areas, the water source supplied to homes and industries may originate from wells, reservoirs through underground pipes or other sources. Often these water supplies contain a high levels of dissolved minerals, agricultural nutrients, and even health hazardous chemicals such as insecticides. Thus, many of those conscious of their drinking and bathing water have installed water purification apparatuses between the inlet water source and the product water in an attempt to remove these undesirable substances. In particular, reverse osmosis water purification apparatuses are more commonly used for removing ionic substances dissolved in the supply water.
Typically, the reverse osmosis water purification apparatus includes a housing having a removable semi-permeable membrane positioned therein which separates the feed water from the product water. These membranes exhibit a propensity for equalizing the concentration of metallic ions dissolved in the water on opposed sides of the membrane. Due to the imbalance of ion concentration, a pressure, commonly known as the osmotic pressure, forces the water through the membrane from the side of lesser ion concentration to the side of greater concentration. Only until concentration equilibrium does the osmotic pressure subside.
This process may be reversed by increasing the fluid pressure on the side of the membrane exhibiting the greater metallic ion concentration which causes the water to flow in the reverse direction; hence the term "reverse osmotic pressure". During this process, the membrane essentially functions as a filter for separating the dissolved substances from the water in which they are suspended.
Under ideal conditions, a reverse osmosis system will filter or reject approximately 95% of the dissolved salts, ferrous particles or other common ions. A rejection ratio of around 70%, however, is acceptable for most applications. Over a period of use or time, the membrane becomes less effective and more susceptible to failure. Often the membrane becomes saturated with contaminants, precipitates, scale, or particulate matter on the water source side of the membrane. Thus, in order for the reverse osmosis water purification apparatus to function properly and efficiently, it is desirable to replace the membrane or service the system when the rejection ratio consistently falls below the acceptable range (i.e., 70%).
Accordingly, it is desirable to monitor the performance of the membrane in order to warn the user when replacement is necessary. One common method of determining membrane performance is to measure the Parts Per Million (PPM) level of dissolved solids in each fluid body (i.e., upstream and downstream) and then calculating the ratio therebetween to determine the percentage difference in ion concentration. Because the concentration of ions in each fluid body is inversely proportional to the resistance of the water, the ion content can easily be calculated by measuring this resistance. Typical monitoring techniques include electronically coupling two electrodes together and passing a current therebetween to measure the resistance, and hence, the PPM concentration.
One problem associated with this technique is that the conductivity or resistivity of the water has been found to be a function of the water temperature. When the water temperature increases, the conductivity increases, while the resistivity decreases. Thus, the resistivity readings vary as the temperature varies. Numerous water quality monitoring devices incorporating this technique of measuring the resistance in the upstream inlet source and the downstream outlet source, have attempted to overcome these conductivity or resistivity variations by introducing various arrays of electronics between the pairs of electrodes.
Typical of such an approach is the monitoring systems disclosed in U.S. Pat. Nos. 4,937,557 to Tucci et al. and to 3,838,774 to Dolan et al. Both devices couple the downstream electrodes to the input resistance of an operational amplifier while coupling the upstream electrodes to the negative feedback loop of the operational amplifier. Because the gain of the operational amplifier is proportional to the ratio between the feedback resistance and the input resistance, this gain represents a continuous measurement of the relative impurity level. This measurement should remain constant during resistance variations. Henceforth, both provide circuitry which compares the gain to a predetermined level; energizing an alarm should the ratio fall below the predetermined level. Although these devices have been adequate to compensate for the resistance variations due to the water temperature changes, they, generally, do not account for the environmental variations which affect the reverse osmosis membrane itself.
The filtering capacity of the membrane has been found to fluctuate due to vacillations in the ion content of the supply water. More importantly, the membranes themselves are sensitive to changes in water temperature, as well as water pressure, even though the membrane may be functioning properly. For example, a substantial temperature increase in the water (not uncommon during the summertime) will increase the capillary size in the semi-permeable membrane leading to an increase in the passage of larger ions. Similarly, a city water pressure decrease, occurring during periods of excessive city use, also results in an increase in capillary size in the membrane. Moreover, these conditions are compounded during the summertime where water usage is generally higher. Thus, these temporary adverse conditions may inconvenience the user by falsely indicating that the semi-permeable membrane's performance has been reduced. Other prior art water quality monitoring apparatus which use the resistance method of measuring the PPM in each fluid body are disclosed in U.S. Pat. Nos. 4,851,818; 4,849,098; 4,806,912; 4,623,451 and 3,990,066.
Although the water quality monitoring systems noted above assess the present performance, adjusting for change in resistivity of the water due to temperature variations, they do not take into account the above-mentioned temporary adverse conditions which may affect the membrane performance itself.
Accordingly, it is an object of the; present invention to provide a water quality monitoring system and method which is improved and more reliable.
It is a further object of the present invention to provide a water quality monitoring system and method which discounts temporary adverse environmental conditions affecting reverse osmosis membrane performance.
Still another object of the present invention is to provide a water quality monitoring system and method which informs the users of the true condition of the membrane and alerts them that replacement is necessary.
Yet, it is another object of the present invention to provide a water quality monitoring system and method which minimizes power consumption and reduces plating or build up of deposits on the electrodes.
Another object of the present invention is to provide a water quality monitoring system and method which can be easily retrofit to existing reverse osmosis water purification apparatuses.
It is a further object of the present invention to provide a water quality monitoring system and method which is durable, compact, easy to maintain, has a minimum number of components, is easy to use, and is economical to manufacture.
The apparatus of the present invention has other objects and features of advantage which will be more readily apparent from the following description of the best mode of carrying out the invention and the appended claims, when taken in conjunction with the accompanying drawing.
Accordingly, there has been a need for a water quality apparatus and method which allows for the temporary adverse conditions which affect reverse osmosis membrane performance. The present invention meets this need.